The present invention relates to a thermal transfer sheet. More particularly, the present invention relates to a thermal transfer sheet for the formation of a high-sensitivity full-color image, which thermal transfer sheet can reproduce a wide range of colors comparable to various types of color printing extensively used on a commercial scale in the art and can provide an image having excellent light fastness and heat resistance.
Various full-color images have hitherto been formed by offset printing, gravure printing, and the like. These conventional printing methods have a problem in that the cost of equipment is high and large space is required, making it impossible to simply carry out color printing in, for example, small workshops, offices, and the like.
In order to solve the above problem, a proposal has been made for a thermal transfer system where thermal transfer sheets for three primary colors are prepared from sublimable (or thermal migration) dyes and then used to transfer the dyes by thermal energy therefrom to form a color image. Such a system requires neither a large printing machine nor other various types of equipment and can easily form a color image, leading to great expectations of the development thereof in the future.
For the above thermal transfer system, a material, on which an image is to be transferred, and a thermal transfer sheet are put on top of the other, and thermal energy corresponding to image information is applied to the resultant assembly on its one side by means of a thermal head, laser, or the like, thereby transferring dyes present on the thermal transfer sheet onto the material on which an image is to be transferred. In this system, intermediate color tones other than three primary colors of yellow, magenta, and cyan are obtained by transferring the dyes in such a manner that dots of two or three colors out of the three primary colors are put on top of the other or one another.
In the conventional thermal transfer system, three primary colors having excellent light fastness and heat resistance have been selected. Certainly, they have excellent light fastness when used alone. However, when one color dot and one or two other color dots are put on top of the other or one another to develop an intermediate color, interaction between the dyes remarkably deteriorates the light fastness, which is a large problem associated with the formation of a full-color image.
Further, the conventional thermal transfer system has a problem that when the formed full-color image is allowed to stand for a long period of time at a relatively high temperature such as in the summer season, dyes constituting the image are diffused, causing fine portions of the image to become unsharp.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer sheet which can provide a full-color image having excellent light fastness and heat resistance in a single-color portion as well as in an intermediate color portion and realize excellent reproduction of a wide range of colors.